1. Field of the Invention
The invention relates to a vehicle powertrain in which provision is made for delivering motive power to vehicle traction wheels in the event of a powertrain control signal failure.
2. Background Art
It is known design practice in the automotive industry to establish a limited power operating strategy for a vehicle powertrain in the event of a powertrain control malfunction. This involves the use of a vehicle creep mode during a so-called “quit-on-the-road” (QOR) event. Upon a loss of a driver-activated acceleration control signal for the powertrain, the powertrain creep mode of operation will permit the operator of the vehicle to maneuver the vehicle off a roadway. This operating strategy is known in the automotive industry as a limited operating strategy (LOS). It is initiated by a loss of signal, which may be identified by the same acronym.
The LOS drive mode is used in situations involving accelerator pedal faults and other faults in which the vehicle system controller will not respond to accelerator pedal movement by the vehicle driver. It is known practice to implement an LOS drive mode in conventional powertrain designs by commanding an engine throttle plate to a fixed throttle angle, thereby commanding a fixed limited power from the engine. This provides a limited wheel torque with a calibrated torque delivery delay.
In the case of a hybrid electric vehicle of the type described, for example, in U.S. Pat. No. 6,994,360, a conventional LOS drive mode is not available because of the powertrain architecture for that type of hybrid electric vehicle. Known methods for implementing an LOS drive mode in response to accelerator pedal signal faults or other similar faults in a hybrid electric vehicle powertrain control system require special strategies to improve the performance of the LOS drive mode.